Our invention pertains to the art of multichannel digital data transmission by time division multiplexing. A typical application of our invention is in the field of intercommunicating systems such as key telephone systems, although we do not wish our invention to be limited to this particular application because the fundamental concepts of our invention obviously lend themselves to application to a greater range of data transmission or reception systems.
The key telephone system has been known which provides intercommunication between a plurality or multiplicity of key operated telephones on the same premises and, possibly, communication with the nationwide telephone network. It has also been known to transmit and receive calls by time division multiplexing, via a time division digital switching circuit, in such key telephone systems. U.S. patent application Ser. No. 792,365 filed Oct. 29, 1985, by Kawamura et al., now U.S. Pat. No. 4,658,397, describes and claims a time division digital data transfer system adaptable for use with a switching circuit in a time division multiplex, digital key telephone system.
The key telephone system disclosed in the noted patent application has several groups of telephones for intercommunication as well as for communication with those of the nationwide network. Each group of telephones are provided with a common multiplexer for sending as many channels of messages over an output highway to the digital switching circuit by time division multiplexing. Each group of telephones are also provided with a common demultiplexer for separating the incoming messages into the individual channels as such messages are fed by mulplexing over an input highway from the switching circuit.
As heretofore constructed, each multiplexer has typically comprised an input buffer, a latch circuit and an output buffer, with the input and output buffers usually taking the form of shift registers. The multiplexers operate to combine the message signals, fed at relatively low speed from the associated telephones, into a time division multiplex signal for transmission at relatively higher speed over the output highway to the switching circuit. The buffers are needed because of the difference in transmission speed on the input and output sides of the multiplexers. Accordingly, the multiplexers must serve the dual purpose of multiplexing and compensating for the difference in transmission speed.
Each demultiplexer has been of like circuit configuration, comprising an input buffer, a latch circuit, and an output buffer. The input buffer takes in the associated channel of intelligence from the multiplex signal delivered at the higher speed from the switching circuit over the input highway. The latch circuit coacts with the output buffer to deliver each channel of intelligence to the associated telephone at the lower speed. Thus the demultiplexers must also perform the dual purpose of demultiplexing and compensating for the difference in transmission speed.
We object to these prior art multiplexer and demultiplexer constructions because each telephone, or any other equivalent terminal device, requires two multiplexing buffer memories and two demultiplexing buffer memories, in addition to latch circuits. Four times as many buffer memories have heretofore been necessary as there are terminal devices. The manufacturing costs of such complex multiplexers and demultiplexers have been prohibitive particularly in data transmission systems employing a large number of terminal devices.